A widely used and successful commercial process for synthesizing acetic acid involves the catalyzed carbonylation of methanol with carbon monoxide. The catalyst contains rhodium and/or iridium and a halogen promoter, typically methyl iodide. The reaction is conducted by continuously bubbling carbon monoxide through a liquid reaction medium in which the catalyst is dissolved. The reaction medium comprises methyl acetate, water, acetic acid, methyl iodide and the catalyst. Conventional commercial processes for carbonylation of methanol include those described in U.S. Pat. Nos. 3,769,329, 5,001,259, 5,026,908, and 5,144,068, the entire contents and disclosures of which are hereby incorporated by reference. Another conventional methanol carbonylation process includes the Cativa™ process, which is discussed in J. H. Jones, “The Cativa™ Process for the Manufacture of Acetic Acid,” Platinum Metals Review, 44 (3): 94-105 (2002), the entire contents and disclosure of which is hereby incorporated by reference.
The carbonylation reactor typically involves an impeller driven mixing system. Impeller driven mixing systems generally incorporate at least one impeller mounted on a rotating shaft. Such systems may be said to have an axial longitudinal flow parallel to the axis of the rotating shaft and/or a radial flow parallel to the radially extending blades mounted on the shaft. The impeller may thus be a radial flow impeller, which projects fluid in a radial direction towards a wall of a tank or chamber in which the impeller is housed, or alternatively, the impeller may be an axial flow impeller, which comprises radially extending blades that are pitched at an angle so as to direct the fluid flow in an axial direction. These and other mechanical agitation mixing methods requiring a rotating shaft are energy intensive, require high capital installations, which includes the shaft to reactor seal system. These reactors utilize mechanical seals, which have finite life cycles and rigorous maintenance requirements.
The reaction solution from the carbonylation reactor contains unreacted starting materials as well as components of the reaction medium, including catalyst. U.S. Pat. No. 5,672,744 discloses a process for the preparation of acetic acid, which comprises carbonylating methanol with carbon monoxide in a carbonylation reactor, in the presence of a reaction fluid comprising a rhodium catalyst, methyl iodide, an iodide salt, methyl acetate and water, while continuously withdrawing the reaction fluid from the reactor and introducing it into a flash zone to separate it into an evaporated part and an unevaporated part.
The carbonylation reaction is exothermic and the heat of the reaction must be regulated to control reaction temperatures. The heat of the reaction typically is transferred to the flasher along with the reaction solution. The flasher operates at a lower pressure than the reactor, which results in a temperature decrease of the reaction solution.
The heat of the reaction may also be recovered. U.S. Pat. No. 6,175,039 discloses removing the heat of reaction from the highly exothermic process by using a steam boiler heated by the pump-around stream. The heat from the reaction may also be transferred to other systems. U.S. Pat. No. 6,114,576 discloses a process for the production of carboxylic acid in the presence of solid catalyst particles that uses the exothermic heat of reaction with high efficiency in recovering the carboxylic acid product. This process uses indirect heat exchange to transfer the heat of reaction to another stream, without the need for an additional heat transfer fluid such as steam. U.S. Pat. No. 7,465,823 discloses a process and system in which a portion of the heat produced during the production of the acetic acid is transferred to the vinyl acetate production and/or purification process and system to facilitate production and/or purification of the vinyl acetate product.
As will be appreciated by one of skill in the art, there is incentive and a need for improvement to existing processes for the production of acetic acid. The present invention provides some such improvements.